1. Field of the Invention
This invention relates to a porous structure of polytetrafluoroethylene (to be referred to as PTFE), and specifically, to a porous structure of PTFE having improved strength.
2. Description of the Prior Art
The porous PTFE structure is utilized in various fields for its excellent thermal stability, chemical resistance, electric insulation, non-tackiness, and lubricity. In particular, a porous PTFE structure produced by stretching has a microfibrous structure composed of very fine fibers and nodes connected to one another by these fibers. Because the pore diameter and porosity of the porous PTFE structure can be changed freely, it is expected to find a wide range of applications as filter materials such as a membrane filters, diaphragms, electrical insulating materials, materials for artificial organs (prostheses) such as an artificial vessels and artificial lungs, an endoscopic tube, etc.
However, since the porous PTFE structure is produced by stretching, the microfibrous structure of PTFE generated by the stretching is strongly oriented in the stretching direction, it has the defect of being readily torn along the stretching direction. Furthermore, the stretched porous PTFE structure has the defect that when a tensile force is applied thereto, the structure is strong in the stretching direction, but is susceptible to deformation at right angles to the stretching direction. In an attempt to remedy this defect, it has been suggested to lay two sheets of porous PTFE structure one over the other so that the stretching directions of the two cross each other at right angles and to bond the two sheets, or to wrap a stretched PTFE sheet about the outside surface of a porous PTFE tubing stretched in the axial direction.
It has now been found that a unitary porous structure of polytetrafluoroethylene can be produced in which the microfibrous structure has one surface having a strong orientation in one direction and another surface having a strong orientation in a direction at right angles to the aforesaid direction, the orientation of said microfibrous structure progressively changing from one surface to the other.